1. Field of the Invention
The present invention relates to an adaptive variable directional antenna suitable for a radio communication system that performs a radio transmission of a digital signal sequence using an orthogonal frequency division multiplexing (OFDM) system.
2. Description of the Related Art
Recently, an information communication system by a digital radio system has attracted much attention. For example, developing advanced digital radio transmission technology is required to realize such as a digital TV broadcasting service using a terrestrial currently operated in the analog system, and a digital multimedia service for the next generation.
In the portable telephone service of the current digital system, transmitted information is low-speed data represented by voice. Therefore, the service is not easily affected by a radio wave propagation environment. On the other hand, with the terrestrial TV broadcasting services and the multimedia communications of the next generation, transmitted information contains a large volume of image data. Therefore, a high radio frequency (high frequency) and a high transmission rate (broad-band) are required to realize these broadcasting services and multimedia communications. A high frequency is subject to deterioration of reception characteristic by fading, thereby causing inferior mobility of the terminal and receiving stations. In addition, a broad band is subject to deterioration of reception characteristic by a multipath (multi-hop transmission), thereby causing difficulty in securing a transmission distance for the terminal and receiving stations as in the case of fading.
Since the service area to be covered by a single broadcasting station is quite large in the terrestrial TV broadcasting service, a receiving image is subject to deterioration by a multipath. Since the scale of the multipath is much larger than the scale of the multipath generated by the above described portable telephone, it is impossible to attain the purpose without employing anti-multipath technology. Normally, an adaptive equalizer is used for anti-multipath technology. However, since the size of a device becomes large with the amount of a multipath corresponding to the size of the device, it has the problem of the mobility and portability of the receiving station which receives data being in a mobile state which can be largely deteriorated.
The multimedia communication system of the next generation aims at seamlessly processing all digitized information such as voice, images, and data and presenting it to the users. Additionally, unlike a TV broadcasting service, different information is required for each user, and therefore, a high-speed radio transmission (broadband radio transmission) should be realized in a small-zone configuration.
Since this system is designed in a small-zone structure, such a grave multipath propagation distortion as in the terrestrial TV broadcasting service is not generated. However, the distortion cannot be completely ignored because the system is established in broadband communication. Furthermore, on condition that it is to be realized that the system is in a zone structure, the frequency utilization should be improved for repetitive use of a frequency, thereby requiring a unit for attenuating the Co-channel interference. However, as in the above described broadcasting system, there is a problem that the mobility and the portability of a receiving terminal station can deteriorate.
As described above, in the radio broadcasting system and radio communication system that hopefully will be realized in the digital system, the most important subject is to realize the broadband communications with the anti-multipath measures and the anti-fading measures taken into account. An orthogonal frequency division multiplexing (OFDM) transmission system capable of transmitting high-quality information with good stability even in a poor multipath propagation environment is regarded with increasing expectation, thereby causing a tendency to employ it for a terrestrial TV broadcasting service and next generation multimedia communications in Japan as well as in Europe.
In the OFDM transmission system referred to as an ultimate multicarrier transmission system, a part of the transmitting signal is duplicated for a guard period as redundancy. The guard period absorbs the multipath propagation following it, thereby preventing grave deterioration. However, the anti-multipath propagation characteristic of the OFDM is not sufficient to successfully avoid deterioration of reception characteristic by the severe frequency selection fading generated by the multipath propagation.
Particularly, in the high-quality image high-definition image) and data transmission expected in the future digital terrestrial TV broadcasting service and the next generation multimedia communications, a transmission should be much higher in quality (lower bit error rate) than in the voice communications. That is, a transmission unit is required to realize much better transmission characteristics. Since the OFDM transmission system is not so stable against the fading generated by a mobile receiving station and a terminal station, or against the Co-channel interference although it has a much better multipath characteristic, it is necessary to find an appropriate method for preventing the above described distortion.
Recently, an adaptive variable directional antenna (adaptive array antenna) is often used as a unit for attenuating the multipath interference, the Co-channel interference, and the fading. The adaptive variable directional antenna is designed to select the direction in space for receiving a radio wave. It can receive only the desired radio wave as long as radio waves (multipath waves and/or common frequency waves) do not arrive as interference from the same direction.
The adaptive variable directional antenna generates an antenna directional beam in a desired radio wave arrival direction by arranging a plurality of antenna elements having the identical characteristics at intervals shorter than the wavelength of radio frequency used for communications in a predetermined array (in weighted linear form, weighted circular form, weighted polygonal form, etc.), and by exciting each antenna element with an appropriate excitation weight, thereby successfully producing an antenna directional null in the unnecessary radio wave arrival direction. Therefore, using an adaptive variable directional antenna, a system resistant to multipath interference and Co-channel interference can be designed.
However, the adaptive variable directional antenna is designed depending on the radio frequency (wavelength) used for communications, and a generated directional pattern (directional beam and directional null) is effective only for a specific radio frequency. Therefore, when the adaptive variable directional antenna is employed for broadband communications, a desired directional pattern cannot be generated for all signal components (frequency components) in the band, thus failing in establishing an effective anti-multipath system.
Especially, an OFDM signal is a broadband signal having tens to thousands of subcarriers. Therefore, even if the adaptive variable directional antenna is used for the OFDM transmission system, only specific subcarriers in a large number of subcarriers can have effective directional pattern, and the other subcarriers have directional patterns ineffective in multipath interference. In the above described situation, an adaptive variable directional antenna, which is effective especially in all communication bands in the OFDM transmission system, is demanded for extremely broad band communications from several to tens MHz in the digital terrestrial TV broadcasting service and the next generation multimedia service to be practically realized in the near future.
Thus, in the next generation multimedia communication systems or terrestrial TV broadcasting systems in which a digital signal sequence is transmitted in the orthogonal frequency division multiplexing (OFDM) system, the anti-multipath propagation characteristic of the OFDM transmission system should be effectively used while attenuating the arrival of multipath interference waves. In addition, the adaptive variable directional antenna is also required to attenuate the arrival of Co-channel interference waves which cause trouble in a cell-domain communication system, and to perform space control of a broadband transmitting/receiving OFDM signal waves.
Particularly, there has been a problem of adaptive variable directional antenna having only one excitation weight for a signal wave with a band width which cannot be in an antenna directivity in the direction in which control should be performed for all frequency components in the OFDM transmission band, thereby failing to produce an antenna directivity effective for all frequency components of received broadband waves.